A conventional clamping device of this type is shown in FIG. 3. FIG. 3 is a partial schematic section of a conventional clamping device S0, with a compact disk shown in clamped condition. Denoted 301 in FIG. 3 is a so-called music reproducing compact disk.
The compact disk 301 is an optical disk with information recorded on one surface thereof, which is basically formed of a resin of good optical transmission such as polycarbonate. The one surface constitutes an information-bearing surface 301a, and the other surface constitutes a label surface 301c. The information-bearing surface 301a has thereon an information-bearing area extending radially from a predetermined inner to outer diameters, in which are recorded information by means of spirally arranged rows of pits.
The label surface 301c has thereon an area in which is coated and formed a film of printing paint (ink) of thermoplastic resin and the like, the area extending radially from a predetermined inner and outer diameters.
Denoted 301b is a center hole having a predetermined inner diameter. The area at an inner radius side around the center hole 301b, on each surface forms a disk clamp area. In the compact disk 301 as shown in this figure, the disk clamp area is located on the inner radius side of the information-bearing area on the information-bearing surface 301a and of the film-coated area on the label surface 301c. The clamping device S0 clamps the compact disk 301 at the disk clamp areas.
On a disk table 402 is placed the information-bearing surface 301a side of the compact disk 301. The disk table 402, in order to level the compact disk 301, is required to have a high flatness at the placement surface and is normally formed of metal such as iron, aluminum or the like, or rigid resin such as a composite resin of polycarbonate with glass fibers, ABS or the like, or a combination thereof.
The disk table 402 has an outer diameter slightly smaller than the inner diameter of the information-bearing area on the information-bearing surface 301a and is provided at its center with a truncated cone projection 402a for engaging in the center hole 301b and positioning the compact disk 301. The disk table 402 is secured to and rotates with a rotating shaft 403 of a motor mounted on a not-shown fixed chassis of the disk clamping device S0.
Denoted 404 is a clamper for pressing the compact disk 301 against the disk table 402, which is formed of metal such as iron, aluminum or the like, resin, or a combination thereof. The clamper 404 is applied, on a surface thereof opposed to the label surface 301c of the compact disk 301, with a damper sheet 404c.
The clamper sheet 404c is ring-shaped, is made of non-woven fabric of synthetic fibers, nylon fibers or polyester fibers, prevents the label surface 301c of the compact disk 301 from getting directly scratched by the clamper 404 and damaged when the compact disk 301 is moved to a predetermined clamping position, and increases the static friction of
the clamper 404 on the compact disk 301 to prevent accidental slipping of the damper on the compact disk.
In FIG. 3, the clamper sheet 404c faces the label surface 301c in the disk clamp area.
The clamper 404 has a rigid steel ball 405 rotatably held at a center of its upper surface, an inverted truncated cone projection 404a on its lower surface which engages in an inverted truncated cone cavity formed at a center of the truncated cone projection 402a, and a circumferential flange 404b.
Denoted 406 is a clamper holder which is rotatably pivoted at one end, via a pin 407, to the fixed chassis of the clamping device S0. The clamper holder 406 is formed at the other end with a hook 406a engageable with the flange 404b.
It is arranged that the clamper holder 406 is rotatable, by means of a not-shown mechanism of the related disk player (not shown), about the pin 407 between the position in FIG. 3 (clamping position) and a later-described position in FIG. 4 (clamp-released position). The clamping operation of the compact disk 301 by the clamper 404 is effected through the movement of the clamper holder 406.
With the clamping device S0, to reproduce the compact disk 301, the clamper holder 406 is rotated by the not-shown mechanism of the disk player to the position in FIG. 3 to press, via the steel ball 405, on the clamper 404 with a predetermined stress, so that the clamper 404 cooperates with the disk table 402 to pressingly hold the compact disk 301 therebetween.
At this time, the clamper sheet 404c and the disk placement surface of the disk table 402 come into contact with the compact disk 301 at the disk clamp area of the latter. Further, at this time, the hook 406a is located spaced from the flange 404b. The clamping device S0 is thus in the position of clamping the compact disk 301.
In this condition, if the disk table 402 is powered by the motor to rotate at a predetermined linear velocity, the clamper 404 rotates with the compact disk 301. In this instance, because of its pressing through the clamper sheet 404c on the compact disk 301, the clamper 404 does not slip on the compact disk 301 and integrally rotates with the disk table 402 and the compact disk 301.
A not-shown pick-up of the disk player may now read recorded information at a predetermined linear velocity from the information-bearing surface 301 a of the compact disk 301.
FIG. 4 shows the clamping device S0 in position having released the compact disk 301. With the clamping device S0, to unclamp the compact disk 301 as shown in FIG. 4 from the clamped condition in FIG. 3, the clamper holder 406 is rotated to bring the hook 406a into engagement with the flange 404b and move the clamper 404 off the compact disk 301 to the position in FIG. 4. The compact disk 301 is thus released from the pressing by the clamper 404 and unclamped.
In recent years, a compact disk player has been widely used not only at home, but also for portable and in-car purposes, with the result that a clamping device used with the player is also used under various severe conditions. Where used in car, for example, it is possible that the temperature inside a car cabin exceeds 80.degree. C. on a mid summer day, and that a compact disk 301 is left in clamped condition by a clamping device for a long time under such conditions.
In ordinary cases, with consideration given to such a use environment, a printing paint (ink) such as will not soften even at elevated temperatures is used for forming a film of paint on a label surface 301c of the compact disk 301.
It has recently been recognized, however, that there exists a disk which uses a printing paint (ink) such as will soften under the above-mentioned high temperature conditions (hereinafter referred to as special disk).
With a special disk 302 of this type, if it has a film of printing paint formed at its disk clamp area as shown in FIG. 5, clamping the special disk 302 will be accompanied with the following problems.
If, for example. the special disk 302 is left clamped by the clamping device S0 in high temperature conditions, the film on the label surface 302c of the special disk 302 will soften, so that the softened film especially at where held by the clamping device S0 (at where in contact with the clamper sheet 404c) penetrates in between fibers of the clamper sheet 404c during the clamping.
The above has been ascertained by an experiment in which the special disk 302 is left clamped by the clamping device S0 for 24 hours in a 50.degree. C. environment.
The thus softened film, with the temperature restoring to room temperature, resolidifies as-is, with the result that portion of the special disk 302 in contact with the clamper sheet 404c adheres to the latter, thereby causing a problem of unsmoothed release of the clamping.